Elastomeric springs have been used in a number of applications to provide an opposing force of compression, torsion, or tension, in response to an applied displacement. The amount of opposing force provided by the elastomeric spring in response to a unit of displacement is known as the spring rate. Elastomeric springs have also been designed to put the elastomer of the spring in tension, shear, or compression in response to the applied displacement. By selecting whether the elastomer is put in tension, shear, or compression, elastomeric springs have been designed for rather linear spring rates and for non-linear spring rates. For example, compression of the elastomer typically results in a non-linear spring rate having a progressive increase in restoring force per unit of displacement as the elastomer is compressed.
Some examples of applications using elastomeric springs include exercise equipment (Whightsil, Sr. U.S. Pat. No. 5,209,461), drive-line couplings (Arlt U.S. Pat. Nos. 4,627,885 and 5,753,463), energy absorbers (Robinson U.S. Pat. No. 6,141,919), flexible pipe joints (Herbert et al. U.S. Pat. No. 4,076,284), and riser tensioner systems (Arlt et al. U.S. Pat. Nos. 5,366,324 and 5,641,248).
Offshore cranes are subject to load variations when making an off-board lift of a payload from a floating vessel such as a supply vessel or materials barge. Severe sea conditions cause heave motion of the floating vessel, and have the effect of placing a transient load upon the crane in excess of the weight of the payload when the payload is lifted off the floating vessel.
Severe sea conditions are considered in industry standards for offshore cranes. For example, the American Petroleum Institute Specification for Offshore Pedestal Mounted Cranes, API-2C, Seventh Edition, March 2012, requires the crane manufacturer to account for sea conditions by using a Dynamic Factor (Cv). The crane capacity for off-board lifts must be de-rated by the Cv. The minimum Cv allowed by the API-2C specification is 1.4 for cranes installed on floating applications and 1.33 for cranes installed on fixed structures. In some cases, Cv will range from 2 to 4. For example, a Cv of 2.8 means that the crane lifting capability at a particular radius from the pedestal mount is one-half of what it could be at the API-2C minimum for the same radius. The Cv is calculated at spaced radii over the range of radii for the crane, and it is a function of three variables. The first variable is a “significant wave height” specified by the customer for the particular marine environment that the crane is designed to operate in. An increase in the “significant wave height” has the effect of raising Cv and reducing the safe working load of the crane. The second variable is the crane geometry. The third variable is crane stiffness taking into account all elements from the hook through the pedestal structure. See, for example, FIG. 8 on page 61 of the API-2C specification, and Section 5.4 on pages 23-31 of the API-2C specification.
Heave compensation of a crane is a method of compensating for the load variations due to heave motion of the crane or the support from which the crane is lifting the payload. Heave compensation attempts to reduce the load variations by raising or lowering the payload to counteract the effect of the heave motions. For example, the objective is for the payload to track a desired reference trajectory in an earth fixed frame without being influenced by heave motions. Heave compensation has employed passive components, active components, and combinations of active and passive components. The passive components have included springs and counter-weights that react to an increase in loading from the payload by reducing the payload acceleration, so that the crane tends to apply a more constant lifting force upon the payload. The active components have included sensors that measure motion of the crane, the payload, or its support, and hydraulic cylinders or winch motors that are actuated in response to the sensor signals in order to counteract the effect of the heave motion. See, for example, Jorg Neupert et al., A Heave Compensation Approach for Offshore Cranes, 2008 American Control Conference, Westin Seattle Hotel, Seattle, Wash., Jun. 11-13, 2008, pages 538-543, American Automatic Control Council, Troy, N.Y.
One kind of passive heave compensator that has been used in the industry has hydraulic or gas cylinders attached to a load block between the hook of the crane and the payload in order to limit dynamic loads. See, for example, Hackman et al. U.S. Pat. No. 4,593,885.